The sunflower (Helianthus annus L.) is one of the most important oil seed crops in the world. Conventional sunflower breeding programs aim at obtaining sunflower cultivars having improved agronomic characteristics such as increased oil and nutritional content, disease resistance and oil quality. To be able to broaden the genetic variation of cultivated sunflower, optimization of tissue culture and transformation techniques is necessary. However, sunflower is known as one of the most recalcitrant species for tissue culture and genetic transformation. Progress in sunflower transformation has been restricted by the limitations of available regeneration systems and problems combining regeneration and transformation within the same cells. Reliable and reproducible transformation methods rely on high transformation rates, efficient regeneration procedures, and available performing selection systems to discriminate and develop dedifferentiated transformed cells (Da{hacek over (g)}üstü, N. et al. (2008) Biotechnol. & Biotechnol Eq. 22: 933-937; Mohamed, Sh. et. al. (2006) Plant Sci. 171: 546-554; Hewezi, T. et al (2002) Plant Mol. Biol. Rep. 20: 335-345).
The protocols currently available for transformation of sunflower plants involve regeneration of plants from transformed explants from mature or immature embryos, or from embryonic parts such as embryonic axes, hypocotyls or cotyledons (U.S. Pat. No. 6,998,516), or depend on regeneration of mature and fertile plants from somatic embryos (U.S. Pat. No. 5,017,491). Also, many of these transformation protocols require extensive preparation of the explants for transformation, or extensive equipment such as that involving particle bombardment of the explants (Malone-Schoneberg, J. et al (1994) Plant Sci. 103: 199-207; U.S. Pat. No. 6,265,638, Knittel, N. et al. (1994) Plant Cell Rep. 14: 81-86, Lucas, O. et al (2000) Mol. Breeding 6: 479-487), have low transformation efficiencies (Schrammeijer, B. et al. (1990) Plant Cell Rep. 9:55-60), involve grafting of transformed shoots onto non-transgenic rootstocks to recover transgenic seeds, result in recovery of chimeric plants (Rao, K. S. (1999) Annals Bot. 83:347-354) or exhibit low regeneration efficiencies of fertile transgenic sunflower plants from the transformed explants.
Methods and compositions for Agrobacterium-mediated high-efficiency transformation and efficient, reproducible regeneration of mature, fertile transgenic sunflower plants are required, and are disclosed herein.